Sensor array beamformer post-processor

ABSTRACT

A novel beamforming post-processor technique with enhanced noise suppression capability. The present beam forming post-processor technique is a non-linear post-processing technique for sensor arrays (e.g., microphone arrays) which improves the directivity and signal separation capabilities. The technique works in so-called instantaneous direction of arrival space, estimates the probability for sound coming from a given incident angle or look-up direction and applies a time-varying, gain based, spatio-temporal filter for suppressing sounds coming from directions other than the sound source direction resulting in minimal artifacts and musical noise.

BACKGROUND

Using multiple sensors arranged in an array, for example microphonesarranged in a microphone array, to improve the quality of a capturedsignal, such as an audio signal, is a common practice. Variousprocessing is typically performed to improve the signal captured by thearray. For example, beamforming is one way that the captured signal canbe improved.

Beamforming operations are applicable to processing the signals of anumber of arrays, including microphone arrays, sonar arrays, directionalradio antenna arrays, radar arrays, and so forth. In general, abeamformer is basically a spatial filter that operates on the output ofan array of sensors, such as microphones, in order to enhance theamplitude of a coherent wave front relative to background noise anddirectional interference. In the case of a microphone array, beamforminginvolves processing output audio signals of the microphones of the arrayin such a way as to make the microphone array act as a highlydirectional microphone. In other words, beamforming provides a“listening beam” which points to, and receives, a particular soundsource while attenuating other sounds and noise, including, for example,reflections, reverberations, interference, and sounds or noise comingfrom other directions or points outside the primary beam. Beamformingoperations make the microphone array listen to given look-up direction,or angular space range. Pointing of such beams to various directions istypically referred to as beamsteering. A typical beamformer employs aset of beams that cover a desired angular space range in order to bettercapture the target or desired signal. There are, however limitations tothe improvement possible in processing a signal by employingbeamforming.

Under real life conditions high reverberation leads to spatial spreadingof the sound, even of point sources. For example, in many cases pointnoise sources are not stationary and have the dynamics of the sourcespeech signal or are speech signals themselves, i.e. interferencesources. Conventional time invariant beamformers are usually optimizedunder the assumption of isotropic ambient noise. Adaptive beamformers,on the other hand, work best under low reverberation conditions and apoint noise source. In both cases, however the improvements possible innoise suppression and signal selection capabilities of these algorithmsare nearly exhausted with already existing algorithms.

Therefore, the SNR of the output signal generated by conventionalbeamformer systems is often further enhanced using post-processing orpost-filtering techniques. In general, such techniques operate byapplying additional post-filtering algorithms for sensor array outputsto enhance beamformer output signals. For example, microphone arrayprocessing algorithms generally use a beamformer to jointly process thesignals from all microphones to create a single-channel output signalwith increased directivity and thus higher SNR compared to a singlemicrophone. This output signal is then often further enhanced by the useof a single channel post-filter for processing the beamformer output insuch a way that the SNR of the output signal is significantly improvedrelative to the SNR produced by use of the beamformer alone.

Unfortunately, one problem with conventional beamformer post-filteringtechniques is that they generally operate on the assumption that anynoise present in the signal is either incoherent or diffuse. As such,these conventional post-filtering techniques generally fail to makeallowances for point noise sources which may be strongly correlatedacross the sensor array. Consequently, the SNR of the output signal isnot generally improved relative to highly correlated point noisesources.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

In general, the present beamforming post-processor technique is a noveltechnique for post-processing a sensor array's (e.g., a microphonearray's) beamformer output to achieve better spatial filtering underconditions of noise and reverberation. For each frame (e.g., audioframe) and frequency bin the technique estimates the spatial probabilityfor sound source presence (the probability that the desired sound sourceis in a particular look-up direction or angular space). It uses thespatial probability for the sound source presence and multiplies it bythe beamformer output for each frequency bin to select the desiredsignal and to suppress undesired signals (i.e. not coming from thelikely sound source direction or sector).

The technique uses so called instantaneous direction of arrival space(IDOA) to estimate the probability of the desired or target signalarriving from a given location. In general, for a microphone array, thephase differences at a particular frequency bin between the signalsreceived at a pair of microphones give an indication of theinstantaneous direction of arrival (IDOA) of a given sound source. IDOAvectors provide an indication of the direction from which a signaland/or point noise source originates. Non-correlated noise will beevenly spread in this space, while the signal and ambient noise(correlated components) will lie inside a hyper-volume that representsall potential positions of a sound source within the signal field.

In one embodiment the present beamforming post-processor technique isimplemented as a real-time post-processor after a time-invariantbeamformer. The present technique substantially improves the directivityof the microphone array. It is CPU efficient and adapts quickly when thelistening direction changes even in the presence of ambient and pointnoise sources. One exemplary embodiment of the present techniqueimproves the performance of a traditional time invariant beamformer 3-9dB.

It is noted that while the foregoing limitations in existing sensorarray beamforming and noise suppression schemes described in theBackground section can be resolved by a particular implementation of thepresent beamforming post-processor technique, this is in no way limitedto implementations that just solve any or all of the noteddisadvantages. Rather, the present technique has a much widerapplication as will become evident from the descriptions to follow.

In the following description of embodiments of the present disclosurereference is made to the accompanying drawings which form a part hereof,and in which are shown, by way of illustration, specific embodiments inwhich the technique may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present disclosure.

DESCRIPTION OF THE DRAWINGS

The specific features aspects, and advantages of the disclosure willbecome better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 is a diagram depicting a general purpose computing deviceconstituting an exemplary system for a implementing a component of thepresent beamforming post-processor technique.

FIG. 2 is a diagram depicting one exemplary architecture of the presentbeamforming post-processor technique.

FIG. 3 is a flow diagram depicting one generalized exemplary embodimentof a process employing the present beamforming post-processor technique.

FIG. 4 is a flow diagram depicting one more detailed exemplaryembodiment of a process employing the present beamforming post-processortechnique.

DETAILED DESCRIPTION

1.0 The Computing Environment

Before providing a description of embodiments of the present Beamformingpost-processor technique, a brief, general description of a suitablecomputing environment in which portions thereof may be implemented willbe described. The present technique is operational with numerous generalpurpose or special purpose computing system environments orconfigurations. Examples of well known computing systems, environments,and/or configurations that may be suitable include, but are not limitedto, personal computers, server computers, hand-held or laptop devices(for example, media players, notebook computers, cellular phones,personal data assistants, voice recorders), multiprocessor systems,microprocessor-based systems, set top boxes, programmable consumerelectronics, network PCs, minicomputers, mainframe computers,distributed computing environments that include any of the above systemsor devices, and the like.

FIG. 1 illustrates an example of a suitable computing systemenvironment. The computing system environment is only one example of asuitable computing environment and is not intended to suggest anylimitation as to the scope of use or functionality of the presentbeamforming post-processor technique. Neither should the computingenvironment be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary operating environment. With reference to FIG. 1 an exemplarysystem for implementing the present beamforming postprocessor techniqueincludes a computing device, such as computing device 100. In its mostbasic configuration, computing device 100 typically includes at leastone processing unit 102 and memory, 104. Depending on the exactconfiguration and type of computing device, memory 104 may be volatile(such as RAM), non-volatile (such as ROM, flash memory, etc.) or somecombination of the two. This most basic configuration is illustrated inFIG. 1 by dashed line 106. Additionally, device 100 may also haveadditional features/functionality. For example, device 100 may alsoinclude additional storage (removable and/or non-removable) including,but not limited to, magnetic or optical disks or tape. Such additionalstorage is illustrated in FIG. 1 by removable storage 108 andnon-removable storage 110. Computer storage media includes volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. Memory104, removable storage 108 and non-removable storage 110 are allexamples of computer storage media. Computer storage media includes, butis not limited to, RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other opticalstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, or any other medium which can be used tostore the desired information and which can accessed by device 100. Anysuch computer storage media may be part of device 100.

Device 100 has a sensor array 118, such as, for example, a microphonearray, and may also contain communications connection(s) 112 that allowthe device to communicate with other devices. Communicationsconnection(s) 112 is an example of communication media. Communicationmedia typically embodies computer readable instructions, datastructures, program modules or other data in a modulated data signalsuch as a carrier wave or other transport mechanism and includes anyinformation delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, RF, infrared and other wireless media. The term computerreadable media as used herein includes both storage media andcommunication media.

Device 100 may have various input device(s) 114 such as a keyboard,mouse, pen, camera, touch input device, and so on. Output device(s) 116such as a display, speakers, a printer, and so on may also be included.All of these devices are well known in the art and need not be discussedat length here.

The present beamforming post-processor technique may be described in thegeneral context of computer-executable instructions, such as programmodules, being executed by a computing device. Generally, programmodules include routines, programs, objects, components, datastructures, and so on, that perform particular tasks or implementparticular abstract data types. The present beam forming post-processortechnique may also be practiced in distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a communications network. In a distributed computingenvironment, program modules may be located in both local and remotecomputer storage media including memory storage devices.

The exemplary operating environment having now been discussed, theremaining parts of this description section will be devoted to adescription of the program modules embodying the present beamformingpost-processor technique.

2.0 Beamforming Post-Processor Technique

In one embodiment, the present beamforming post-processor technique is anon-linear post-processing technique for sensor arrays, which improvesthe directivity of the beamformer and separates the desired signal fromnoise. The technique works in so-called instantaneous direction ofarrival space to estimate the probability of the signal coming from agiven location (e.g., look-up direction in angular space) and uses thisprobability to apply a time-varying, gain-based, spatio-temporal filterfor suppressing sounds coming from other non-desired directions otherthan the estimated sound source direction, resulting in minimalartifacts and musical noise.

2.2 Exemplary Architecture of the Present Beamforming Post-ProcessorTechnique.

One exemplary architecture of the present beamforming post-processortechnique 200 is shown in FIG. 2. This architecture 200 consists of aconventional beamformer 202 which receives inputs from an array ofsensors, such as, for example, an array of microphones 204. The outputof the beamformer 202 is input into a post-processor 206, which consistsof a spatial filtering module 210 and a spatial probability estimationmodule 208 which employs an instantaneous direction of arrivalcomputation. The spatial probability estimation module 208 estimates theprobability that the desired signal originates from a given direction,θ_(S), using the inputs from the array of sensors. This probability isthen multiplied by the beamformer output in the spatial filtering module210, to provide the desired sound source signal with an improved signalto noise ratio 212.

2.3 Exemplary Process Employing the Present Beamforming Post-ProcessorTechnique.

One very general exemplary process employing the present post-processorbeamforming technique is shown in FIG. 3. As shown in FIG. 3, box 302,signals of a sensor array in the frequency domain are input into astandard beamformer. A beamformer output is computed as a function ofthe input signals divided into frequency bins and an index of timeframes (box 304). The probability that the desired signal originates agiven direction θ_(S) is computed using an instantaneous direction ofarrival computation (box 306). This probability is multiplied by thebeamformer output (box 308) to produce the desired signal with anenhanced signal to noise ratio (box 310).

More particularly, a more detailed exemplary process employing thepresent beamforming post-processor technique for a microphone is shownin FIG. 4. The audio signals captured by the microphone arrayx_(i)(t),i=1 . . . (M−1), where M is the number of microphones, aredigitized using conventional analog to digital (A/D) conversiontechniques, breaking the audio signals into frames (boxes 402, 404). Thepresent beamforming post-processor technique then converts thetime-domain signal x_(i)(n) to the frequency-domain (box 406). In oneembodiment a modulated complex lapped transform (MCLT) is used for thispurpose, although other conventional transforms could equally well beused. One can denote the frequency domain transform as X_(i) ^((n))(k),where k is the frequency bin, n is the index of the time-frame (e.g.,frame), and i is the microphone (where i is 1 to M)).

The signals in the frequency domain, X_(i) ^((n))(k), are then inputinto a beamformer, whose output represents the optimal solution forcapturing an audio signal at a target point using the total microphonearray input (box 408). Additionally, the signals in the frequency domainare used to compute the instantaneous direction of arrival of thedesired signal for each angular space (defined by incident angle orlook-up angle (box 410)). This information is used to compute thespatial variation of the sound source position in presence of Noise(N(0,λ_(IDOA)(k))), for each frequency bin. The IDOA information and thespatial variation of the sound source in the presence of Noise is thenused to compute the probability density that the desired sound sourcesignal comes from a given direction, θ, for each frequency bin (box412). This probability is used to compute the likelihood that for afrequency bin k of a given frame the desired signal originates from agiven direction θ_(S) (414). If desired this likelihood can alsooptionally be temporally smoothed (box 416). The likelihood, smoothed ornot, is then used to find the estimated probability that the desiredsignal originates from direction θ_(S). Spatial filtering is thenperformed by multiplying the estimated probability the desired signalcomes from a given direction by the beamformer output (box 418),outputting a signal with an enhanced signal to noise ratio (box 420).The final output in the time domain can be obtained by taking theinverse-MCLT (IMCLT) or corresponding inverse transformation of thetransformation used to convert to frequency domain (inverse Fouriertransformation, for example), of the enhanced signal in the frequencydomain (box 422). Other processing such as encoding and transmitting theenhanced signal can also be performed (box 424).

2.4 Exemplary Computations

The following paragraphs provide exemplary models and exemplarycomputations that can be employed with the present beamformingpost-processor technique.

2.4.1 Modeling

A typical beamformer is capable of providing optimized beam design forsensor arrays of any known geometry and operational characteristics. Inparticular, consider an array of M microphones with a known positionsvector {right arrow over (p)}. The microphones in the array sample thesignal field in the workspace around the array at locationsp_(m)=(x_(m),y_(m),z_(m)):m=0, 1, . . . , M−1. This sampling yields aset of signals that are denotes by the signal vector {right arrow over(x)}(t,{right arrow over (p)}).

Further, each microphone m has a known directivity pattern, U_(m)(f,c),where f is the frequency and c={φ,θ, ρ} represents the coordinates of asound source in a radial coordinate system. A similar notation will beused to represent those same coordinates in a rectangular coordinatesystem, in this case, c={x,y,z}. As is known to those skilled in theart, the directivity pattern of a microphone is a complex function whichprovides the sensitivity and the phase shift introduced by themicrophone for sounds coming from certain locations or directions. Foran ideal omni-directional microphone, U_(m)(f, c)=constant. However, themicrophone array can use microphones of different types and directivitypatterns without loss of generality of the typical beamformer.

2.4.1.1 Sound Capture Model

Let vector {right arrow over (p)}={p_(m)m=0,1 . . . −1}; denote thepositions of the M microphones in the array, wherep_(m)=(x_(m),y_(m),z_(m)). This yields a set of signals that one candenote by vector {right arrow over (x)}(t,{right arrow over (p)}). Eachsensor m has known directivity pattern U_(m)(f,c), where c={φ,θ, ρ}represents the coordinates of the sound source in a radial coordinatesystem and f denotes the signal frequency. It is often preferable toperform signal processing algorithms in the frequency domain becauseefficient implementations can be employed.

As is known to those skilled in the art, a sound signal originating at aparticular location, c, relative to a microphone array is affected by anumber of factors. For example, given a sound signal, S(f) originatingat point c, the signal actually captured by each microphone can bedefined by Equation (1), as illustrated below:

X _(m)(f,p _(m))=D _(m)(f,c)S(f)+N _(m)(f)  (1)

where the first term on the right-hand side,

$\begin{matrix}{{D_{m}\left( {f,c} \right)} = {\frac{^{{- {j2\pi}}\; f\frac{{c - p_{m}}}{v}}}{{c - p_{m}}}{A_{m}(f)}{U_{m}\left( {f,c} \right)}}} & (2)\end{matrix}$

represents the delay and decay due to the distance from the sound sourceto the microphone ∥c—p_(m)∥, and v is the speed of sound. The termA_(m)(f) is the frequency response of the system preamplifier/ADCcircuitry for each microphone, m, S(f) is the source signal, andN_(m)(f) is the captured noise. The variable U_(m)(f,c) accounts formicrophone directivity relative to point c.

2.4.1.2 Ambient Noise Model

Given the captured signal, X_(m)(f,p_(m)) the first task is to computenoise models for modeling various types of noise within the localenvironment of the microphone array. The noise models described hereindistinguish two types of noise: isotropic ambient nose and instrumentalnoise. Both time and frequency-domain modeling of these noise sourcesare well known to those skilled in the art. Consequently, the types ofnoise models considered will only be generally described below.

The captured noise N_(m)(f,p_(m)) is considered to contain two noisecomponents: acoustic noise and instrumental noise. The acoustic noise,with spectrum denoted with N_(A)(f) is correlated across all microphonesignals. The instrumental noise, having a spectrum denoted by the termN_(I)(f), represents electrical circuit noise from the microphonepreamplifier, and ADC (analog/digital conversion) circuitry. Theinstrumental noise in each channel is incoherent across the channels,and usually has a nearly white noise spectrum N_(I)(f). Assumingisotropic ambient noise one can represent the signal, captured by any ofthe microphones, as a sum of infinite number of uncorrelated noisesources randomly spread in space:

$\begin{matrix}{N_{m} = {{N_{A}{\sum\limits_{l = 1}^{\infty}{{D_{m}\left( c_{l} \right)}{{\mathbb{N}}\left( {0,{\lambda_{l}\left( c_{l} \right)}} \right)}}}} + {N_{l}{{\mathbb{N}}\left( {0,\lambda_{I}} \right)}}}} & (3)\end{matrix}$

Indices for frame and frequency are omitted for simplicity. Estimationof all of these noise sources is impossible because one has a finitenumber of microphones. Therefore, the isotropic ambient noise is modeledas one noise source in different positions in the work volume for eachframe, plus a residual incoherent random component, which incorporatesthe instrumental noise. The noise capture equation changes to:

N _(m) ^((n)) =D _(m)(c _(n))N(0,λ_(N)(c _(n)))+N(0,λ_(NC))  (4)

where c_(n) is the noise source random position for n^(th) audio frame,λ_(N)(c_(n)) is the spatially dependent correlated noise variation(λ_(N)(c_(n))=const ∀c_(n) for isotropic noise) and λ_(NC) is thevariation of the incoherent component.

2.4.2 Spatio-Temporal Filter

The sound capture model and noise models having been described, thefollowing paragraphs describe the computations performed in oneembodiment of the present beamforming post-processor technique to obtaina spatial and temporal post-processor that improves the quality of thebeamformer output of the desired signal. The following paragraphs arealso referenced with respect to the flow diagram shown in FIG. 4.

2.4.2.1 Instantaneous Direction Of Arrival Space

In general, for a microphone array, the phase differences at aparticular frequency bin between the signals received at a pair ofmicrophones give an indication of the instantaneous direction of arrival(IDOA) of a given sound source. IDOA vectors provide an indication ofthe direction from which a signal and/or point noise source originates.Non-correlated noise will be evenly spread in this space, while thesignal and ambient noise (correlated components) will lie inside ahyper-volume that represents all potential positions of a sound sourcewithin the signal field.

To provide an indication of the direction a signal or noise sourceoriginates from (as indicated in FIG. 4, box 410), one can find theinstantaneous Direction of Arrival (IDOA) for each frequency bin basedon the phase differences of non-repetitive pairs of input signals. For Mmicrophones these phase differences form a M−1 dimensional space,spanning all potential IDOA. If one defines an IDOA vector in this spaceas

$\begin{matrix}{{\Delta (f)}\overset{\Delta}{=}\left\lbrack {{\delta_{1}(f)},{\delta_{2}(f)},\ldots \mspace{11mu},{\delta_{M - 1}(f)}} \right\rbrack} & (5)\end{matrix}$

where δ_(i)(f) is the phase difference between channels 1 and i+1:

δ_(i)(f)=arg(X ₁(f))−arg(X _(i+1)(f))l−{1, . . . , M−1}  (6)

then the non-correlated noise will be evenly spread in this space, whilethe signal and ambient noise (correlated components) will lay inside ahypervolume that represents all potential positions c={φ,θ, ρ} of asound source in real three dimensional space. For far field soundcapture, this is a M−1 dimensional hypersurface as the distance ispresumed to approach infinity. Linear microphone arrays can distinguishonly one dimension—the incident angle, and the real space is representedby a M−1 dimensional hyperline. For each frequency a theoretical linethat represents the positions of sound sources in the angular range of−90 degrees to +90 degrees can be computed using Equation (5). Theactual distribution of the sound sources is a cloud around thetheoretical line due to the presence of an additive non-correlatedcomponent. For each point in the real space there is a correspondingpoint in the IDOA space (which may be not unique). The opposite is notrue: there are points in the IDOA space without corresponding point inthe real space.

2.4.2.2 Presence of a Sound Source.

For simplicity and without any loss of generality, a linear microphonearray is considered, sensitive only to the incident angle θ-direction ofarrival in one dimension. The incident angle is defined by adiscretization of space. For example, in one embodiment a set of anglesis defined that is used to compute various parameters—probability,likelihood, etc. Such set can, for example, be in from −90 to +90degrees every 5 degrees. Let Ψ_(k)(θ) denote the function that generatesthe vector Δ for given incident angle θ and frequency bin k according toequations (1), (5) and (6). In each frame, the k^(th) bin is representedby one point Δ_(k) in the IDOA space. Consider a sound source at θ_(S)with its correspondence in IDOA space at Δ_(S)(k)=Ψ_(k)(θ_(S)). Withadditive noise, the resultant point in IDOA space will be spread aroundΔ_(S)(k).

Δ_(S+N)(k)=Δ_(S)(k)+N(0,λ_(IDOA)(k)).  (7)

where N(0,λ_(IDOA)(k)) is the spatial movement of Δ_(k) in the IDOAspace caused by the correlated and non-correlated noises.

2.4.2.3 Space Conversion

The distance from each IDOA point to the theoretical in IDOA space iscomputed as a function of incident angle space, as shown in FIG. 4, box412. The conversion from the distance from an IDOA point to thetheoretical hyperline in IDOA space into the incident angle space (realworld, one dimensional in this case) is given by:

$\begin{matrix}{{\mathrm{\Upsilon}_{k}(\theta)} = \frac{{\Delta_{k} - {\Psi_{k}(\theta)}}}{\frac{{\Psi_{k}(\theta)}}{\theta}}} & (8)\end{matrix}$

where ∥Δ_(k)−Ψ_(k)(θ)∥ is the Euclidean distance between Δ_(k) andΨ_(k)(θ) in IDOA space,

$\frac{{\Psi_{k}(\theta)}}{\theta}$

are the partial derivatives, and γ_(k)(θ) is the distance of observedIDOA point to the points in the real world. Note that the dimensions inIDOA space are measured in radians as phase difference, while γ_(k)(θ)is measured in radians as units of incident angle. This computationprovides the distance between each IDOA point and the theoretical lineas a function of the incident angle for each frequency bin and eachframe.

2.4.2.4 Estimation of the Variance in Real Space

As shown in FIG. 4, box 414, in order to compute the probability thatthe sound source originates from a given incident angle, one must havethe conversion from distance to the theoretical hyperline in IDOA spaceto distance into the incident angle space given by Equation (7) and thenoise properties.

Analytic estimation in real-time of the probability density function fora sound source in every frequency bin is computationally expensive.Therefore the beamforming post-processor technique estimates indirectlythe variation λ_(k)(θ) of the sound source position in presence of noiseN(0,λ_(IDOA)(k)) from Equation (7). Let λ_(k)(θ) and γ_(k)(θ) be a K×Nmatrix where K is the number of frequency bins and N is the number ofdiscrete values of the incident or direction angle of the microphone.Variation estimation goes through two stages. During the first stage arough variation estimation matrix

(θ,k) is built. If θ_(min) is the angle that minimizes γ_(k)(θ), onlythe minimum values in the rough model are updated:

_(k) ^((n))(θ_(min))=(1−α)

_(k) ^((n−1))(θ_(min))+αγ_(k)(θ_(min))²  (9)

where γ is estimated according to Eq. (8),

$\alpha = \frac{T}{\tau_{A}}$

is the adaptation time constant, T is the frame duration). During thesecond stage a direction-frequency smoothing filter H(θ,k) is appliedafter each update to estimate the spatial variation matrixλ(θ,k)=H(θ,k)*

(θ,k). Here it is assumed a Gaussian distribution of the non-correlatedcomponent, which allows one to assume the same deviation in the realspace towards the incident angle, θ.

2.4.2.5 Likelihood Estimation

As shown in FIG. 4, box 416, a likelihood estimation that the desiredsignal comes from a given incident angle is computed using the IDOAinformation and the variation due to noise. With known spatial variationλ_(k)(θ) and the distance of the observed IDOA points to the points inthe real world, γ_(k)(θ), the probability density for frequency bin k tooriginate from direction θ is given by:

$\begin{matrix}{{{p_{k}(\theta)} = {\frac{1}{\sqrt{2{{\pi\lambda}_{k}(\theta)}}}\exp \left\{ \frac{{\mathrm{\Upsilon}_{k}(\theta)}^{2}}{2{\lambda_{k}(\theta)}} \right\}}},} & (10)\end{matrix}$

and for a given direction, θ_(S) the likelihood that the sound sourceoriginates from, this direction for a given frequency bin is:

$\begin{matrix}{{{\Lambda_{k}\left( \theta_{S} \right)} = \frac{p_{k}\left( \theta_{S} \right)}{p_{k}\left( \theta_{\min} \right)}},} & (11)\end{matrix}$

where θ_(min) is the value which minimizes p_(k)(θ).

2.4.2.6 Spatio-Temporal Filtering

Besides spatial position, the desired (e.g., speech) signal has temporalcharacteristics and consecutive frames are highly correlated due to thefact that this signal changes slowly relatively to the frame duration.Rapid change of the estimated spatial filter can cause musical noise anddistortions in the same way as in gain based noise suppressors. As shownin FIG. 4, box 418, to reflect the temporal characteristics of thespeech signal, temporal smoothing can optionally be applied. For a givendirection, the absence/presence of speech can be modeled with twostates: S₀ and S₁. The sequence of frequency bin states is modeled asfirst-order Markov process. Then the pseudo-stationary property of thedesired (e.g., speech) signal can be represented byP(q_(n)=S₁|q_(n−1)=S₁) with the following constraint:P(q_(n)=S₁|q_(n−1)=S₁)>(q_(n)=S₁) where q_(n) denotes the state of n-thframe as either S₀ or S₁. By assuming that the Markov process is timeinvariant, one can use the notation

$a_{ij}\overset{\Delta}{=}{{P\left( {q_{n} = {\left. H_{j} \middle| q_{n - 1} \right. = H_{j}}} \right)}.}$

Based on the formulations above, a recursive formula for signal presencelikelihood for given lookup direction in n^(th) frame Λ_(k) ^((n)) isobtained as:

$\begin{matrix}{{{\Lambda_{k}^{(n)}\left( \theta_{S} \right)} = {\frac{a_{01} + {a_{11}{\Lambda_{k}^{({n - 1})}\left( \theta_{S} \right)}}}{a_{00} + {a_{10}{\Lambda_{k}^{({n - 1})}\left( \theta_{S} \right)}}}{\Lambda_{k}\left( \theta_{S} \right)}}},} & (12)\end{matrix}$

where a_(ij) are the transition probabilities, Λ_(k)(θ_(S)) is estimatedby Equation (11), and Λ_(k) ^((n))(θ_(S)) is the likelihood of having asignal at direction θ_(S) for n^(th) frame. As shown in FIG. 4, box 420,this likelihood can be converted to a probability and spatial filteringcan be performed by multiplying the probability that the desired signalcomes form a given direction times the beamformer output. Morespecifically, conversion to probability gives the estimated probabilityfor the speech signal to originate from this direction:

$\begin{matrix}{{P_{k}^{(n)}\left( \theta_{S} \right)} = {\frac{\Lambda_{k}^{(n)}\left( \theta_{S} \right)}{1 + {\Lambda_{k}^{(n)}\left( \theta_{S} \right)}}.}} & (13)\end{matrix}$

The spatio-temporal filter to compute the post-processor output Z_(k)^((n)) (for all frequency bins in the current frame) from the beamformeroutput Y_(k) ^((n)) is:

Z _(k) ^((n)) =P _(k) ^((n))(θ_(S)) ·Y _(k) ^((n)),  (14)

i.e. the signal presence probability is used as a suppression.

It should also be noted that any or all of the aforementioned alternateembodiments may be used in any combination desired to form additionalhybrid embodiments. For example, even though this disclosure describesthe present beamforming post-processor technique with respect to amicrophone array, the present technique is equally applicable to sonararrays, directional radio antenna arrays, radar arrays, and the like.Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.The specific features and acts described above are disclosed as exampleforms of implementing the claims.

1. A compute-implemented process for improving the directivity andsignal to noise ratio of the output of a beamformer employed with asensor array, comprising inputting signals of sensors of a sensor arrayin the frequency domain defined by frequency bins and frames in time;computing a beamformer output as function of the input signals dividedinto frequency bins and frames in time; dividing a spatial regioncorresponding to a working space of the sensor array into a plurality ofincident angle regions, and for each frequency bin and incident angleregion, computing the probability that the desired signal occurs at agiven incident angle region using an instantaneous direction of arrivalcomputation; and spatially filtering the beamformer output bymultiplying the probability that the desired signal occurs at a givenincident angle region by the beamformer output.
 2. Thecomputer-implemented process of claim 1 wherein the input signals of thesensor array in the frequency domain are converted from the time domaininto the frequency domain prior to inputting them using a ModulatedComplex Lapped Transform (MCLT).
 3. The computer-implemented process ofclaim 1 wherein the sensors are microphones and wherein the sensor arrayis a microphone array.
 4. The computer-implemented process of claim 1wherein the sensors are one of: sonar receivers and wherein the sensorarray is a sonar array; directional radio antennas and the sensor arrayis a directional radio antenna array; and radars and wherein the sensorarray is a radar array.
 5. The computer-implemented process of claim 1wherein the instantaneous direction of arrival computation for eachfrequency bin is based on the phase differences of the input signalsfrom a pair of sensors.
 6. The computer-implemented process of claim 1wherein spatially filtering the beamformer output attenuates signalsoriginating from directions other than the direction of the desiredsignal.
 7. A computer-implemented process for improving the signal tonoise ratio of one or more signals from sensors of a sensor array,comprising: inputting signals from microphones of a microphone array inthe frequency domain; dividing the input signals into frequency bins andframes; computing a beamformer output from the input signals of themicrophone array; dividing a spatial region corresponding to a workingspace of the sensor array into a plurality of incident angle regions;for each frequency bin and incident angle region, estimating aninstantaneous direction of arrival point which provides an estimationfrom which direction a signal or noise source originates based on thephase differences of pairs of input signals from two differentmicrophones; computing the distance from each instantaneous direction ofarrival point to a theoretical line of instantaneous direction ofarrival as a function of the incident angle for each frequency bin;computing the variation due to noise in the estimation from whichdirection a signal or noise source originates for each frequency bin fora set of incident angle ranges; performing a likelihood estimation thata desired signal comes from a given incident angle using the computedvariation due to noise; and converting the likelihood that the desiredsignal comes from a given incident angle into a probability that thedesired signal comes from a given incident angle; and multiplying theprobability that the desired signal comes from a given incident angle bythe beamformer output to output a signal with an enhanced signal tonoise ratio.
 8. The computer-implemented process of claim 7 furthercomprising temporally smoothing the likelihood estimation that a desiredsignal comes from a given incident angle prior to converting thelikelihood to a probability.
 9. The computer-implemented process ofclaim 8 wherein temporally smoothing the likelihood estimation that adesired signal comes from a given incident angle comprises: modeling theabsence or presence of the desired signal and frequency bin state overconsecutive frequency bins and frames; and for a given frequency bin andframe, weighting the likelihood estimation based on the absence orpresence of the desired signal and the frequency bin state in previousframes and frequency bins.
 10. The computer-implemented process of claim9 wherein the frequency bin state is modeled using a first order Markovprocess.
 11. The computer-implemented process of claim 7 furthercomprising convert the enhanced output signal from the frequency domaininto the time domain.
 12. The computer-implemented process of claim 7further comprising at least one of: encoding the enhanced output signal;and transmitting the encoded enhanced output signal; and transmittingthe enhanced output signal.
 13. A computer-readable medium havingcomputer-executable instructions for performing the process recited inclaim
 7. 14. A system for improving the signal to noise ratio of asignal received from a microphone array, comprising: a general purposecomputing device; a computer program comprising program modulesexecutable by the general purpose computing device, wherein thecomputing device is directed by the program modules of the computerprogram to, capture audio signals in the time domain with a microphonearray; convert the time-domain signals to frequency-domain and frequencybins using a converter; input the signals in the frequency domain into abeamformer and computing a beamformer output wherein the beamformeroutput represents the optimal solution for capturing an audio signal ata target point using the total microphone array input; estimate theprobability that a desired signal comes from a given incident angleusing an instantaneous direction of arrival computation; and output anenhanced signal with a greater signal to noise ratio by taking theproduct of the beamformer output and the probability estimation that thedesired signal comes from a given incident angle.
 15. The system ofclaim 14 wherein the instantaneous direction of arrival computation foreach frequency bin is based on the phase differences of the inputsignals from a pair of microphones.
 16. The system of claim 14 whereinthe instantaneous direction of arrival computation comprises: for eachfrequency bin and incident angle from a microphone, estimating aninstantaneous direction of arrival point which provides an estimationfrom which direction a signal or noise source originates; computing thedistance from each instantaneous direction of arrival point to atheoretical line of instantaneous direction of arrival as a function ofthe incident angle from a microphone for each frequency bin; computingthe variation due to noise in the estimation from which direction asignal or noise source originates for each frequency bin for a set ofincident angles; performing a likelihood estimation that a desiredsignal comes from a given incident angle direction using the computedvariation due to noise; and converting the likelihood that the desiredsignal comes from a given incident angle direction into a probabilitythat the desired signal comes from a given incident angle direction. 17.The system of claim 16 wherein the likelihood estimation that a desiredsignal comes from a given incident angle using the computed variationdue top noise is temporally smoothed prior to converting the likelihoodinto the probability.
 18. The system of claim 14 wherein the beamformeris a time-invariant beamformer.
 19. The system of claim 14 wherein theenhanced signal with a greater signal to noise ratio is computed andoutput in real time.
 20. The system of claim 14 wherein the modules toestimate the probability that a desired signal comes from a givenincident angle using an instantaneous direction of arrival computationand the module to output an enhanced signal with a greater signal tonoise ratio by taking the product of the beamformer output and theprobability estimation that the desired signal comes from a givenincident angle form a post-processor that attenuates signals originatingfrom directions other than the direction of the desired signal to outputa signal with an enhanced signal to noise ratio.